1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to methods and receiver architectures in client terminals for signal level measurements during network acquisition.
2. Description of Related Art
Client terminals used in wireless communication systems are required to search for the network, acquire the network information, register and camp on to the network. The aforementioned process is collectively called “network acquisition.” The network acquisition process would normally take place in different scenarios that include but are not limited to powering on the client terminal, an attempt to obtain service after a loss of network coverage (e.g., a dropped call due to a “dead spot” in the network), and upon roaming from one network to another. In many situations the signal level measurements performed as part of the network acquisition process may take a long time, such as up to several seconds or longer. Such delays may adversely impact client terminal performance and user satisfaction.
An example of a cellular wireless network is provided in FIG. 1. As shown in the figure, cellular-type wireless communication system 10 comprises elements such as a client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. As illustrated, the communication path from the base station (“BS”) to the client terminal direction is referred to herein as the downlink (“DL”) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (“UL”). In some wireless communication systems the client terminal or mobile station (“MS”) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
As shown in FIG. 2, client terminal/MS 12 typically contains a baseband subsystem 16 and a radio frequency (“RF”) subsystem 18. Memory 20, such as an external memory, is shown connected to the baseband subsystem 16. The baseband subsystem 16 normally includes a micro controller unit (“MCU”) 22, a signal processing unit (“SPU”) 24, data converters 26, peripherals 28, power management 30, and memory 32 as shown in FIG. 3. The SPU 24 may be a digital signal processor (“DSP”), hardware (“HW”) accelerators, co-processors or a combination of the above. Normally the overall control of the baseband subsystem 16 is performed by software running on the MCU 22 and the processing of signals is done by the SPU 24.
Analog to digital converters (“ADCs”) convert a received analog signal into digital for the baseband system to process it. Similarly, digital to analog converters (“DACs”) convert the processed baseband digital signal into analog for transmission. The ADCs and DACs are collectively referred to herein as “data converters” 26. The data converters 26 can either be part of the baseband subsystem 16 or the RF subsystem 18. Depending on the location of the data converters 26, the interface between the two subsystems will be different. The location of the data converters 26 does not alter the overall function of the client terminal.
An RF subsystem 18 normally contains a receiver, a transmitter, a synthesizer, a power amplifier, an antenna, and other components. An RF subsystem 18 for a time division duplex (“TDD”) system is shown in FIG. 4. Receiver section 34 performs the task of converting the signal from RF to baseband. It includes mixers 36, filters 38, low noise amplifiers (“LNAs”) 40 and variable gain amplifiers (“VGAs”) 42. Transmitter section 44 performs the task of converting the baseband signal up to the RF. It includes mixers 46, filters 48, and gain control through VGAs 50. Power amplification of the transmit signal is typically done by a separate power amplifier (“PA”) unit 52 but is considered part of the transmit RF chain. In some architectures, some of the components of the receiver and transmitter can be shared. As shown, the receiver section 34 and the transmitter section 44 are coupled to an antenna 54 via a transmit/receive switch 56. Synthesizer 58 is also shown as coupling to the receiver section 34 and the transmitter section 44.
Often, multiple receive and transmit chains are used in wireless communication systems to improve performance. The performance improvement can be in terms of better coverage, higher data rates, multiplexing of multiple users on the same RF channel at the same time, or some combination of the above. FIG. 5 illustrates an RF subsystem 60 with two RF receive chains.
As shown, RF subsystem 60 includes a transmitter 62, a synthesizer 64, and a pair of receivers 661 and 662. One of the receivers, 661, and the transmitter 62 are coupled to a first antenna 681 via transmit/receive switch 70. The other receiver, 662, is connected to a second antenna 682. In this type of RF subsystem, whenever the multiple receive chains are used they are all tuned to the same RF channel.
While such approaches may provide an acceptable level of performance, it is desirable to provide improved signal level measurement techniques that improve the network acquisition process.